Methods and Apparatus For Cochlear Implant Signal Processing

ABSTRACT

A cochlear implant processing strategy increases speech clarity and higher temporal performance. The strategy determines the power spectral component within each channel, and dynamically selects or de-selects the channels through which a stimulation pulse is provided as a function of whether the spectral power of the channel is high or low. “High” and “low” are estimated relative to a selected spectral power, for example. The selected spectral power can be estimated by signal average or mean, or by other criteria. Once a selection of the channels to stimulate has been made, the system can decide that only those channels are stimulated, and stimulation is removed from the other channels. The selected channels are the ones on which the spectral power is above the mean of all the available channels. Fewer channels are stimulated at any time and the contrast of the stimulation is enhanced. Also, the temporal resolution increases as the number of channels that must be stimulated on a given frame decreases. This way, the channels which are presented to the patient are fewer in number and contain more temporal information.

RELATED APPLICATIONS

The present application is a DIVISIONAL of U.S. patent application Ser.No. 11/096,402, by Gene Y. Fridman, filed on Apr. 1, 2005, now issued asU.S. Pat. No. ______, and entitled “Methods and Apparatus For CochlearImplant Signal Processing,” the contents of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTIONS

The present invention relates to cochlear implant systems and methods,for example cochlear implant signal processing methods and apparatus,and speech processing and stimulation strategies used by such cochlearimplant systems.

BACKGROUND OF THE INVENTIONS

Cochlear implant systems provide the sensation of sound to those who areprofoundly deaf. Unfortunately, the clarity of the sound that isperceived is not always as good as desired.

U.S. Pat. No. 5,626,629, and U.S. Pat. No. 5,601,617, both of whichpatents are incorporated herein by reference, teach or use some speechprocessing and stimulation strategies that may be used by a cochlearimplant system, such as the CLARION or C-II cochlear stimulation systemsavailable from Advanced Bionics Corporation, of Sylmar, Calif. Onecommon speech processing strategy used in the prior art is asimultaneous analog stimulation (SAS) strategy, wherein more than onechannel may provide stimulation at the same time. Another common speechprocessing strategy used and known in the art is continuous interleavedsampling (CIS) strategy. U.S. Pat. No. 6,289,247, also incorporatedherein by reference, teaches other types of speech processing andstimulation strategies that may be used by a cochlear implant system.U.S. Pat. No. 5,597,380, and U.S. Pat. No. 5,271,397 are likewiseincorporated herein by reference.

SUMMARY

Dynamic selection of the number of channels to stimulate can providegreater sound clarity. Once a selection of the channels to stimulate hasbeen made, stimulation can be removed from the other channels.Stimulation can be applied to the selected channels only, if desired.

In one configuration, the selected channels are the ones on which thespectral power is above the mean of all the available channels. In thisexample, few channels get stimulated at any one time for a given frame,and the contrast of the stimulation is enhanced. The contrast isimproved further because the perceived loudness on the fewer number ofchannels will increase due to faster presentation rate. Also, thetemporal resolution will increase as the number of stimulated channelsis decreased.

Further, because the selected channels are the ones on which thespectral power is above a threshold, e.g., above the mean, or above theaverage, or above some other measure of the spectral power on all of thechannels, the selection of channels often is not static. Rather, theselection can be dynamic based on the spectral power in the channels.

One configuration of a stimulation system applies stimulation to theareas of the cochlea which correspond in the desired way to spectralpower, such as the selected spectral power. Stimulation may be removedfrom all other locations along the cochlea corresponding to channelshaving a low spectral power, for example below the selected spectralpower.

Cochlear implants having one or more of the characteristics describedabove may offer increased speech clarity and higher temporalperformance. They may also offer increased speech clarity withoutconsuming excessive power.

The present invention advantageously provides an increase in perceivedSNR (signal-to-noise ratio) by removing stimulation from low powerchannels. Further, the invention provides an increase in spectralcontrast since fewer channels receive a higher pulse rate. Additionally,the invention provides an increase in temporal resolution since theintegration frame is shorter for a smaller number of channels.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present inventions will be moreapparent from the following more particular description thereof,presented in conjunction with the following drawings wherein:

FIG. 1 shows a cochlear stimulation system;

FIG. 2 shows a typical biphasic stimulation waveform generated by animplantable cochlear stimulator (ICS);

FIG. 3 illustrates the signal flow through a cochlear stimulation systemin accordance with the present inventions;

FIG. 3A is a schematic of a processor assembly used on a platform forthe stimulation system; and

FIG. 4 depicts an illustration of how the present inventions may processsignals to provide stimulation only on those channels of high spectralpower.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION

The following description is of the best mode presently contemplated forcarrying out one or more aspects of the present inventions. Thisdescription is not to be taken in a limiting sense, but is made merelyfor the purpose of describing the general principles of the invention.The scope of the invention should be determined with reference to theclaims.

In one exemplary apparatus and methods, improved speech clarity can beachieved by only stimulating the locations of the cochlea whichcorrespond to high spectral power, namely spectral power above a definedspectral power. Additionally, stimulation can be removed from all otherlocations along the cochlea with low spectral power, namely spectralpower below the selected spectral power. “Low spectral power” and “Highspectral power” are defined here as being that spectral power that isbelow and above the selected spectral power, respectively. In one aspectof the inventions, the selected spectral power is estimated by thesignal average. In the examples described herein, the “signal average”is the sum of the channel signals divided by the total number ofchannels.

A representative cochlear stimulation system with which the presentinvention may be used is described in U.S. Pat. No. 5,603,726, whichpatent is incorporated herein by reference. Other cochlear stimulationsystems with which the present invention may be used are found in U.S.Pat. Nos. 6,308,101; 6,219,580; and 6,272,382; which patents are alsoincorporated herein by reference.

FIG. 1 shows a typical cochlear stimulation system 10 comprising aspeech processor portion 12 and a cochlear stimulation portion 14. Thecochlear stimulation portion 12 is usually implanted, and includes animplantable cochlear stimulator (ICS) 16 and a cochlear lead 18. Thelead 18 includes a multiplicity of electrode contacts thereon (notvisible in FIG. 1) through which electrical stimulation pulses,generated by the ICS 16, are applied to selected locations or areas ofthe cochlea.

The speech processor portion 12 includes a speech processor (SP) 20 anda microphone 22. The microphone 22 may be physically connected to the SP22, or connected through an appropriate wireless link 21. The microphone22 senses acoustic sound and transduces it to an electrical signal. Theelectrical signal from the microphone has different intensities as afunction of the loudness of the audio signal that is sensed. Theelectrical signal from the microphone 22 is then processed by the SP 20in accordance with a selected speech processing strategy. Based on thetype of processing strategy employed, appropriate control signals aregenerated and sent to the ICS 16 over link 24. The ICS 16 responds tothese control signals by generating appropriate stimulation signals thatare applied to tissue at various locations along the inside of thecochlea through the electrode contacts located near the distal end ofthe lead 18.

Typically, the speech processor portion 12 of the cochlear stimulationsystem 10 is external (not implanted), and the link 24 between the SP 20and the IPG 16 is a transcutaneous link. However, it is to be understoodthat parts of the speech processor portion 12 may also be implanted. Ina fully-implantable cochlear stimulation system, such as is described inthe U.S. Pat. No. 6,308,101, all of the speech processor portion 12 isimplanted. When both the SP portion 12 and the cochlear stimulationportion 14 are implanted, the SP 20 and the ICS 16 may reside inrespective housings, as shown in FIG. 1, or the circuitry associatedwith both the SC 20 and the ICS 16 may be combined into a singlehousing.

A biphasic pulse of the type that is generated by the ICS 16 in responseto the control signals received from the SP 20 is shown in FIG. 2. Ingeneral, the amplitude and/or pulse width (PW) of the pulses may bevaried to adjust the magnitude of the stimulus. Also, the frequency, orstimulation rate, at which the pulses are generated may be controlled,as needed.

A preferred platform for launching the present invention is shown inU.S. Pat. No. 6,219,580, previously incorporated herein by reference.Some features associated with that platform are shown in the signal flowdiagram of FIG. 3. Additionally, a schematic of a processor assemblyused on the platform is shown in FIG. 3A, showing a physicalpartitioning of the ICS2 portion of the platform described andillustrated in FIG. 14 of U.S. Pat. No. 6,219,580. The ICS2 consists ofelectronic circuitry that fits inside a hermetically sealed, U-shapedceramic case 300, e.g., of the type disclosed in U.S. Pat. No.4,991,582, incorporated herein by reference. The package design may bethe same as is used by the ICS described in the '726 patent, previouslyreferenced. The power and telemetry coils 302, and the back telemetrycoil 304, and all circuitry are mounted on a ceramic hybrid 306 insideof the case 300. The majority of the circuitry is integrated into customintegrated circuits (ICs). Two IC's are employed—one analog IC 308 andone digital IC 310. Discrete components are used as necessary, e.g.coupling capacitors C_(C). Attachment of the circuitry to the sixteenexternal electrodes and one indifferent (reference) electrode is througha bulkhead connector 312 at one end of the case. (Note that Electrodesare numbered 1 through 16, with 1 the most apical and 16 the mostbasal.) Provision for an additional indifferent electrode and twostapedius electrodes are also made in the ICs.

As seen in FIG. 3, the signal generated by the microphone 22 is splitinto frequency bands by a bank of bandpass filters 40-0, 40-1, . . .40-k connected in parallel. Each bandpass filter 40 receives themicrophone signal. Each bandpass filter 40 has a center frequency thatallows signal frequencies within a specified band to pass therethrough.The bandpass filter 40-0, for example, allows relatively low frequencysignals to pass through. The bandpass filter 40-k, on the other hand,allows only high frequency signals to pass through. The bank of bandpassfilters is an example of apparatus that can divide an incoming audiosignal into channels.

The signals in each frequency band are then subjected to envelopedetectors 50-0, 50-1 . . . 50-k. Each of these envelope detectors 50senses the spectral power component of the signal in its respectivefrequency band. This spectral power component is represented in FIG. 3as the signals E₀, E₁, . . . E_(K). The average of these signals E₀, E₁,. . . E_(K) is dynamically determined. The average of the power of thespectral channels can be determined through appropriate processingcarried out on the ICS2 (FIG. 3A). This average allows selection of aselected spectral power, which can then be used to select which of thesignals E₀, E₁, . . . E_(K) represents “low” spectral power, and whichof the signals represents “high” spectral power. The envelope detectoris an example of apparatus that can determine the spectral power of asignal. The selected spectral power is an example of a threshold thatcan be used to differentiate between high and low spectral powersignals, and the threshold can be determined through appropriateprocessing carried out on the ICS2 (FIG. 3A). A selector circuit 60allows only those signals having “high” spectral power to be sent on tothe ICS for stimulus generation. The channels having “low” spectralpower are de-selected, i.e., removed so that stimulation pulsescorresponding to the channels having “low” spectral power is effectivelyturned off. These low spectral power channels will be effectively zerochannels, because they will be turned off, or those channels will be setat zero or have no pulses applied for those channels. The selectorcircuit 60 is an example of an apparatus for selecting channels havingspectral power above a threshold value. The selector circuit can beimplemented in the ICS2 (FIG. 3A).

Further, the signals of “high” spectral power that pass through theselector circuit 60 are sequenced using sequencer 64 so that the stimuligenerated by the ICS 16 are applied sequentially only on the non-zero(spectral power) channels. Acoustic-to-electrical mapping of the signalsis further carried out with mapping circuits 70-0, 70-1, . . . 70-k,which mapping further conditions the signals that are applied to theelectrodes on the lead 18. A biphasic stimulus pulse is then applied onthe non-zero channels in sequence as controlled by the sequencer 64 andas conditioned by appropriate mapping circuits 70. The mapping circuitsare an example of an apparatus for sequentially applying electricalstimuli only to the electrodes of channels having a spectral power abovea threshold value.

Because the spectral power in each channel changes dynamically as afunction of the acoustic signals sensed through the microphone 22, thenon-zero channels through which a stimulus, or stimuli, are applied alsochanges dynamically. However, for any cycle of the sequencer 64, therewill be some zero channels on which no stimulus will be provided, andsome non-zero channels on which a biphasic stimulus pulse is applied.The biphasic stimulus provides a loudness associated with the variousparameters of the stimulation pulse train, such as the amplitude, pulsewidth of the pulses, and the time between pulses. —We refer to theperceived loudness on a given channel as intensity. Intensity iscontrolled by the spectral power of that channel. The intensity in someapplications will be the combination of the amplitude and the pulsewidth and number of pulses per unit time, but it should be understoodthat intensity for purposes of the present discussion may be manifestedin other ways, for example amplitude only with relatively constant pulsewidth, or pulse width with relatively constant amplitude. For theexample illustrated in FIG. 3, the spectral power is non-zero only inthe 0th channel and the kth channel. All of the other channels are zero.Thus, a biphasic pulse 72 is applied to electrode 0 on lead 18, and abiphasic pulse 74 is applied to electrode k on the lead 18. Electrode 0,corresponding to channel 0, which represents the channel having thelowest frequency components, is located distally near the end of thelead 18 so that when the lead is inserted into the cochlea thiselectrode 0 is close to those nerve cells deep in the cochlea thatrecognize lower frequency signals. Electrode k, corresponding to channelk, which represents a channel having higher frequency components, islocated more proximally on the lead 18 so that when the lead is insertedinto the cochlea this electrode k is close to those nerve cells closerto the entrance of the cochlea that recognize higher frequency signals.

The operation of the invention is depicted in signal processingillustration of FIG. 4. Note that FIG. 4 is divided into five rows,labeled (A), (B), (C), (D), and (E). Each row represents a differentexample of a signal processing condition.

In row (A) of FIG. 4, the speech power spectrum has peaks at about 800and 2500 Hz, as seen in the Speech Power Spectrum graph 80A. However,these peaks are not sharp peaks, meaning that the spectral power isspread over many of the channels. This creates a power spectral spreadin each of eight channels as illustrated in “Envelope Output” chart 82A.This is basically a chart of the signals E₀, E₁, . . . E_(K), (see FIG.3), where k=8 in this example. The average of these signals E₀, E₁, . .. E_(K) is determined and is used as a threshold. The average is takenas equal to Sum(Ej)/K, where Sum is the conventional sum of elements(Ej), “j” is the channel number, and “K” is the total number ofchannels, or “k” in this example, where k=8. This threshold, or“threshold value” 83A, is then used as the selected spectral power, andused to identify those channels above the threshold value and thosechannels below the threshold value. For the example of this row (A),five of the eight channels—channels 2, 3, 4, 5 and 6—have spectral powersignals above the threshold 83A. Hence, these five channels areselected, as seen in the Dynamic Peak Selection Output chart 88A, andbiphasic pulses are applied sequentially to these five channels, asillustrated in the Stimulation Pattern chart 90A.

The speech power spectrum in rows (B), (C), (D) and (E) of FIG. 4 showsthat the spectral peaks, all of which are at approximately 800 and 2500Hz, become increasingly sharper. Thus, for example, in Row (E), only twoof the eight channels—channels 3 and 6—have spectral power above theaverage 83E. Hence, only these two channels are selected, as seen in theDynamic Peak Selection Output chart 88(E), and biphasic pulses areapplied sequentially to just these two channels, as illustrated in theStimulation Pattern chart 90(E). Note also the rate at which the pulsesare applied to these two channels is faster than the rate would be ifmore channels were selected. This increases the temporal resolution forthe stimulated channels since the time between stimuli is shorter. Thus,in general, the temporal resolution increases since the integrationframe is shorter for a smaller numbers of channels.

Thus, it is seen that in operation for one aspect of the present signalprocessing method, the speech processing strategy operates by splittingthe incoming signal (obtained from the microphone 22, or equivalent)into k frequency bands or channels. The spectral power component of thesignal present in each frequency band is determined. The average (ormean, or other suitable collective measure) of these spectral powercomponents, E₀, E₁, . . . E_(K), is determined and is used as athreshold value. Only those channels having a spectral power componentabove the threshold value is selected for stimulation as a non-zerochannel. The other channels are de-selected, as zero spectral powerchannels, and no stimulus is applied on these zero channels. The stimuliare then applied sequentially only on the selected channels.

While the mean is one example of a criterion for identifying a selectedspectral power to use, for example as a threshold value, other criteriamay be used as well. Other examples include other statistical methods,such as using variance to determine a threshold value or a combinationof the average and variance to determine a threshold value. Otherexamples include using a weighted average, such as where the weight maybe dynamically assigned or where it is assigned as a function of knownspeech spectra. Dynamic assignment of a weighting factor may includeweighting based on relative or absolute amplitude, based on noise levelas may be determined dynamically, for example, or other weightingmethods. Additionally, weighting may be applied to incoming signalsbefore they are analyzed or they may be applied to the threshold valueor other determinant before calculating which channels will be non-zeroand which will be assigned zero values. Other examples for identifying aspectral power to use include identification of the median, oridentification of the median with a weight factor applied. Therefore,identification of the selected spectral power may be considered to bebased on some function, f(E₀, E₁, E₂, E₃, . . . . E_(K)), hereafterf(E), as desired, which function is then used to select the non-zerochannels. The function may also be a function of time, which will bedesignated as f(Et), indicating that the function is based preferably onboth the spectral power but also on time “t”. The function f(E) will beused when indicating that the spectral power is based on the channelenergies regardless of any dependence on time.

The function f(Et) can be used or applied dynamically, as a function ofon-going speech patterns, and need not be static for a given time periodor static for a given user. For example, the mean in the examplediscussed herein can be determined for the channels on a frame-by-framebasis or on some other basis selected by the designer or technician. Themean (or other criterion) can be determined at regular intervals, orwhen a selected event occurs, such as when signal levels rise or fallbeyond a set level. For present purposes, using a function to select aselected spectral power, different than a previously selected spectralpower, more than once during the lifetime of the device will beconsidered dynamically determining the selected spectral power.Therefore, “t” in f(Et) can be as large as the device lifetime, and assmall as a frame or less. Additionally, the number of channels selectedfor stimulation can be varied. The selection criteria may be the same asdescribed herein, for example, and such selection will allow the numberof channels that are stimulated to be changed as a function of time aswell. Therefore, the number of channels for stimulation may be selected.At a later time, one or more channel signals may be evaluated, such asby applying a suitable function to the spectral power values for thechannel (or each channel desired), and thereafter identifying thosechannels with spectral power values that exceed a selected value. Thenumber of channels may then be increased or decreased as desired. Thenumber of channels may be decreased or increased dynamically based onthe desired criteria or criterion.

Advantageously, the present apparatus and methods can be used toincrease the perceived signal-to-noise ratio (SNR) because thestimulation from the low spectral power channels can be identified andremoved. Moreover, the spectral contrast increases since fewer channelsreceive a higher pulse rate. Additionally, the temporal resolutionincreases since the integration frame can be made shorter for smallernumber of channels. As a further advantage, the power consumption of thecochlear stimulation system can be less than when using simultaneousspeech processing strategies, such as SAS.

Because the present invention may operate using less power than an SASstrategy, SAS users who choose the present strategy would have theoption of using a behind-the-ear (BTE) speech processor, which consumesless power than the body-worn speech processors.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

1. A method for signal processing in a cochlear implant, the methodcomprising: separating an incoming audio signal into a plurality offrequency channels, wherein each channel includes a channel signalrepresenting a portion of the audio signal corresponding to the channel;determining a spectral characteristic of the channel signal for at leastone of the channels; applying a function f(E) to the channel signal anddetermining a value for the channel signal as a function of f(E);selecting channel signals from the plurality of channels that exceed thevalue; and designating the selected channels whose signals exceed thevalue as channels to be used for stimulation.
 2. The method of claim 1wherein applying a function includes determining the mean of at leasttwo channel signals.
 3. The method of claim 1 wherein applying afunction includes determining a mean for all channels in the pluralityof frequency channels.
 4. The method of claim 3 wherein designating theselected channels includes designating the selected channels whosespectral powers exceed the mean of spectral powers for each of theplurality of the channels.
 5. The method of claim 1 wherein applying afunction to the channel signal is carried out dynamically.
 6. The methodof claim 1 wherein the function f(E) comprises a spectral power functionthat determines how much spectral power is present in a selected channelsignal.
 7. The method of claim 6 wherein the function f(E) comprises aspectral power function that also applies a weight factor.
 8. The methodof claim 1 wherein the function f(E) comprises a spectral power functionthat is also a function of time, f(Et), where f(Et) indicates that thefunction is based on both spectral power and on time, “t”.
 9. A methodfor signal processing in a cochlear implant, the method comprising:separating an incoming audio signal into a plurality of channels,wherein each channel includes a channel signal representing a portion ofthe audio signal corresponding to the channel; applying a function f(E)to the channel signal and determining a value for the channel signal asa function of f(E); selecting channel signals from the plurality ofchannels that exceed the value; and designating the selected channelswhose signals exceed the value as channels to be used for stimulation.10. The method of claim 9 wherein applying a function includesdetermining the mean of at least two channel signals.
 11. The method ofclaim 9 wherein applying a function includes determining a mean for allchannels in the plurality of frequency channels.
 12. The method of claim11 wherein designating the selected channels includes designating theselected channels whose spectral powers exceed the mean of spectralpowers for each of the plurality of the channels.
 13. The method ofclaim 9 wherein applying a function to the channel signal comprisesapplying the function dynamically.
 14. The method of claim 9 wherein thefunction f(E) comprises a spectral power function that determines howmuch spectral power is present in a selected channel signal.
 15. Themethod of claim 14 wherein the function f(E) comprises a spectral powerfunction that also applies a weight factor.
 16. The method of claim 9wherein the function f(E) comprises a spectral power function that isalso a function of time, f(Et), where f(Et) indicates that the functionis based on both spectral power and on time, “t”.
 17. A speechprocessing strategy for use within a cochlear stimulation system, thecochlear stimulation system having a speech processor portion thatprocesses incoming audio signals and an implantable cochlear stimulation(ICS) portion, coupled to the speech processor portion, that applieselectrical stimuli through cochlear electrodes to surrounding cochleartissue as a function of selected characteristics of the incoming audiosignal, the speech processing strategy comprising: signal processingcircuitry that divides the incoming audio signal into separate channelsas a function of frequency, each channel having at least one electrodeassociated therewith through which electrical stimuli is applied tosurrounding cochlear tissue; a first detector that detects spectralpower associated with that portion of the audio signal present in eachchannel through the use of a prescribed spectral power function, f(E),applied to the channel signal; a second detector that dynamicallydetects when the spectral power associated with a channel signal exceedsa prescribed threshold value; and a pulse generator circuit thatgenerates electrical stimuli and dynamically applies the stimuli only tothe electrodes of those channels where the spectral power exceeds theprescribed threshold.
 18. The speech processing strategy of claim 17wherein the spectral power function, f(E), detects spectral power usingone or more statistical methods selected from the group comprisingdetermining a mean, determining an average, determining a variance, andweighting,
 19. The speech processing strategy of claim 17 wherein theprescribed threshold used by the second detector is determined using oneor more statistical methods selected from the group comprisingdetermining a mean, determining an average, determining a variance, andweighting,